Conventionally, there is well known an engine having a supercharger and an EGR device. The supercharger compresses air and sends the air to the engine compulsorily. Compared with a normal engine with the same displacement, the engine having the supercharger can generate larger output and torque.
The EGR device returns a part of exhaust gas to the intake side. The EGR device has an EGR valve in an EGR passage connecting the exhaust side to the intake side.
The EGR valve is opened and closed so as to regulate amount of the exhaust gas returned to the intake side. Accordingly, the EGR device reduces the combustion temperature of the engine so as to suppress generation of nitrogen oxides (NOx) which cause photochemical smog.
However, at the transient state of the engine, delay of response of the supercharger or throttle of intake and exhaust caused by the supercharger itself increases the EGR amount excessively, whereby intake air amount is in short extremely against injection amount so as to generate smoke. For preventing the generation of smoke, several EGR amount control means are disclosed.
The Japanese Patent Laid Open Gazette Hei. 09-021356 discloses an engine with a supercharger which revises EGR gas amount so as to prevent increase of smoke by excessive response at sudden acceleration. The Japanese Patent Laid Open Gazette Hei. 09-079092 discloses an engine that the EGR rate is reduced to about 10% at the time of shift from idling to acceleration state. The Japanese Patent Laid Open Gazette 2002-161791 discloses an engine that the EGR valve is fully closed at the time of sudden acceleration.
Each of the above patent literature and the conventional art is suppression or interruption of the EGR at the acceleration state (that is, at the time of increasing engine rotation speed) and does not intend for suppression or interruption of the EGR at the time of reducing engine rotation speed.